Continuous casting of tubes



Aug. 27, 1968 D. c. YEARLEY 3,398,780

I CONTINUOUS CASTING OF TUBES Filed July 1, 1965 5 Sheets-Sheet 1 FIG. la

VIBRATOR M A O A, Ill

\ A VIBRATOR INVENTOR. DOUGLAS C. YEARLEY ATTORN .75.

Aug. 27, 1968v D. c. YEARLEY CONTINUOUS CASTING OF TUBES 5 Sheets$heet 2 Filed July 1, 1965 w m P.

.d 0 Y. mm W m m m 1 mm 2 m m w 4 F 6 M p. w 2 7 Zr v Aug. 27, 1968 o. c. YEARLEY CONTINUOUS CASTING OF TUBES 5 Sheets-Sheet 3 Filed July 1, 1965 INVENTOR. DOUGLAS C. YEARLEY FIG.3

United States Patent ABSTRACT OF THE DISCLOSURE Continuous casting of tubes by solidfying molten material as it is withdrawn from a solidification zone defined by the insertion of a central mandrel into the exit aperture of a liquid zone to create an annular aperture of adjustable dimensions to control the characteristics of the cast product.

SPECIFICATION This invention relates to a method and apparatus for the continuous casting of hollow metal tubes.

Continuous casting of tubes comprises the uninterrupted flow of molten metal to one end of a casting mold and the removal of heat from the metal to solidify it as it is formed into the desired sectional shape during passage of the metal through the mold. Tubing is cast continuously by the use of an inner core or mandrel positioned within a cylindrical mold or die. The molten metal is supplied under pressure or by gravity feed to the annular space between the mandrel and the cylindrical mold. The mold is cooled to solidify the metal, and cast tube is drawn from the mold by rollers or the like which grip the outer surface of the tubing.

In general, the relatively slow rates of continuously casting tubing in prior apparatus and the poor quality of such tubing are serious problems which heretofore have made it economically impractical to continuously cast tubing for direct fabrication into commercial tubing. Those problems are due in part to adhesion of the metal to the surfaces of the mold and the mandrel and to shrinking of the metal on the mandrel during the cooling and solidifying phase of the process. These difficulties have persisted despite various counteracting measures which have been attempted, such as vibrating the mold and providing the mandrel with a taper in the direction of the metal movement through the mold.

Other factors which have precluded extensive use of continuous tube casting for direct fabrication to commercial tubing include the difiiculty in starting the tube casting process, the tendency for the cast tube to have an intolerable degree of eccentricity and lack of control over the solidification process in the mold during the casting operation.

Control over the solidification process is important since the process of solidification, including the rate of solidification and its location, determines the properties of the resulting tubing such as surface quality, tensile strength, elongation, grain size, etc.; and lack of control of that process will, of course, give poor properties in the tubmg.

This invention provides an improved apparatus for the continuous casting of tubes and a new method for such casting, which will allow for the manufacture of high quality tubes on a commercial and economical basis. It eliminates start-up difliculties, provides a concentric tubing of good quality which may be rapidly and economically fabricated into commercial tubing. Another impor- "ice tant aspect of this invention is that it permits the molten metal used to form the tubing to be fed to the casting mold by gravity without any need for metering that flow or the use of a feed spout to deliver and position the metal in the mold.

According to the present invention, the mandrel is mounted for vertical movements relative to a mold Which is fed from a molten pool of metal maintained in a reservoir above the mold. The latter, in turn, is mounted for vertical vibratory movements in which it is guided by means for preventing lateral displacement of the mold. In starting the tube casting operation, molten metal is first fed to the mold from the overlying reservoir while the mandrel is withdrawn, whereby solid rod is drawn initially from the exit or lower end of the mold; and then, as the feed of molten metal is continued, the mandrel is lowered through the reservoir so that the tapered lower end of the mandrel enters the mold to a depth sufficient to effect transition from rod to tube casting. The mandrel is guided in its vertical movements by guide means overlying the reservoir and adjustable laterally to effect accurate centering of the mandrel relative to the mold.

In the preferred form of the new apparatus, the mold and the mandrel are provided with separate cooling systems. Also, to prevent adhesion of the metal during casting and to improve the grain structure of the cast tube, the mold and the mandrel are vibrated vertically either in unison or separately.

The method and apparatus of my invention have been used successfully to produce good quality phosphorous deoxidized copper tubes having overall diameters of 1 /2 to 4 inches with wall thicknesses ranging from 0.17 to 4.45 inches, at speeds in excess of 2 feet per minute and with an error of concentricity of less than 0.01 inch.

The invention may be better understood from the following description of a specific example as illustrated by the accompanying drawings, in which:

FIGS. 1a and 1b are together a vertical view of a preferred form of the apparatus of my invention;

FIG. 2 is an enlarged vertical sectional view of the mold and the crucible forming the feed reservoir, as shown in FIG. 1a; and

FIG. 3 is an enlarged vertical sectional view of a modified form of the mandrel having a separate cooling system.

Referring to the drawings, particularly FIG. 1a, metal M in molten form is introduced from an external source, not shown, by means of a launder or trough 11 into a generally cylindrical crucible 12 of graphite mounted on a vertically movable platform 13. The latter has a generally rectangular shape in plan view and any conventional means (not shown) may be used to support it.

The platform 13 has attached to its intermediate its right front and rear corners and left front and rear corners supports 36 and 36', respectively. Struts 37 and 37' reinforce cross bracket 35 which is attached at each of its ends to supports 36 and 36'. These members support the upper portion of the apparatus, as more fully hereinafter explained.

The platform has attached to its a vibrator (shown schematically) which may be of any conventional design, preferably one to impart a vertical sinusoidal vibratory motion to the platform 13 and thus to the casting apparatus. By rigid connection (by means not shown) of supports 36 and 36' with the bar 30 both the mandrel and the mold may be vibrated by the same means. Without that connection the mandrel may be used without vibrating it or it may be vibrated independently by attaching a conventional vibrator, such as an ultrasonic transducer, shown schematically, to bar 30.

In practice, I have found it suitable to provide the vertical vibrations to the casting apparatus with an amplitude of about to inch at a frequency ranging from a few hundred to several thousand cycles per minute.

Heat from crucible 12 is insulated from platform 13 by insulating bricks 16. The crucible is enclosed in an induction coil 17 carrying water as the coolant and supported in place by refractory lining 18 of material such as silica or the like which is rammed between the coil and the outer surface of the crucible. The open end of the crucible is suitably provided with a plate 19 of ring form. Mounted within the crucible 12 through a central aperture in the bottom thereof is a guide bearing 20 also of graphite and having generally radial and sloped feed ports 21 to allow passage of the liquid metal into a cylindrical casting mold 22. The latter is closely surrounded by the lower portion of bearing 20, and the upper portion of this bearing forms a close sliding fit around a graphite mandrel 23 so as to center the mandrel in the graphite casting mold 22.

The mandrel 23 is supported from an overhead support 25 from which depends (shown schematically) hoist means 26 for raising and lowering the mandrel. The hoist 26 may be hand or motor driven, but preferably is a conventional hand operated worm gear which allows precise positioning of the mandrel. Suitably attached to hoist 26 is a depending eye bolt 27 connected to one end of a strain measuring device 28. Elongation of device 28, by tensile strain on the depending mandrel 23, is detected by strain gauges 29 suitably mounted on strain device 28 and connected electrically to a conventional indicator (not shown) for indicating the amount of strain on the device 28 and thus the downward force exerted on mandrel 23 during the tube casting operation. As such strain measuring devices are well known in the art, further description of the device 28 and its associated elements is unnecessary.

The lower end of strain measuring device 28 is connected by a bar member 30 to a guide post 31. The latter is held in vertical alignment with the guide bearing 20 and mold 22 by a steel guide bearing 32 into which precision-machined bronze bushings 33 have been pressured. Guide bearing 32 is mounted in an alignment box 34 secured to cross bracket 35. Orientation of the guide bearing 32 in the mounting box 34 is adjustable by bolts 38 for angular positions with the vertical and is adjustable horizontally by radial bolts such as bolt 39. Guide bearing 32 is locked in its adjusted position by a cover plate 40 and bolts 41.

Mandrel 23 is rigidly connected to guide post 31 by a connecting pin 43. A cross bolt- 44 connects one end of the connecting pin 43 to the guide post 31 while a similar cross bolt 45 connects the mandrel 23 to the other end of connecting pin 43. To prevent wobble between the guide post 31 and the mandrel 23, two lock-nuts 46 and 47 are threaded on pin 43 and tightened against the guide post 31 and mandrel 23, respectively.

Liquid metal in the casting mold 22 is solidified in the mold portion lying within a cooling jacket 50. The latter is provided at the bottom with an annular flange plate 52 through which extend a plurality of studs 51 depending from platform 13. Jacket is held in position by nuts 53 threaded on studs 51 and bearing against the bottom of flange plate 52.

The metal solidifying between mold 22 and mandrel 23 is withdrawn as tube 54, which passes downward into a quench chamber 55 (FIG. 1b). Water passing through jacket 50, along the path indicated by the arrows in FIG. 1a, flows downward to chamber 55 to serve as the quenching liquid. The quench chamber 55 is drained through exit port 56 to a recirculating water system with means to cool the water (not shown), the cool Water being circulated to jacket 50 through inlet port 75 (FIG. 2). The cast tube discharged through the bottom of quench tank 55 by way of a sealed outlet comprising a boss 57 welded to the quench tank 55 and welded to a flange 58. Several rubber gaskets 59 are firmly, clamped by a ring 60 to the welded flange 58 to form a seal which wipes the surface of the cast tube 54 as it discharges. The tube is pulled downwardly through seal 59 by withdrawal rolls 61 mounted on a fixed stand 62 and tightened into position against the tube by adjusting screws 63 having operating handles 63a.

Referring now to FIG. 2, graphite .guide bearing 20 is centrally mounted in crucible 12 by a press-taper fit, as at 64, and is held in place by a graphite nut 65 tightened on threads on the lower end of the guide bearing. A sleeve 66 is provided on the inner surface of the graphite guide bearing 20 and may be easily replaced in the event of Wear to insure a close-tolerance slide fit between the mandrel 23 and guide bearing 20. Mandrel 23 is positioned in mold 22 by operation of hoist 26 and adjustment of box 34 (which may be adjusted upon wearing of graphite guide bearing 20), to effect concentric solidification, as at 67, at the desired portion of the tapered section 68 of mandrel 23. Thus, the cast tube 54 will have essentially a uniform wall thickness.

Mandrel 23 is cylindrical for substantially its entire length from its upper end to a distance just short of its lower end. This cylindrical portion is indicated in FIG. 2 by numeral 231. The next lower portion 232 is given a slight taper and the next lower portion 233 has a greater taper of about twice the rate of the taper of portion 232. The tip portion 234 is not necessarily tapered, although it is preferable to provide it with a point to allow the mandrel to penetrate easily the surface of the molten metal as the mandrel is lowered into the mold. In one design for a machine casting a tube with a 3-inch O.D., the mandrel was 2 feet long, portion 231 was 20 inches long with a diameter of 2.35 inches, portion 232 was 2.5 inches long with a taper of 1 while portion 233 was provided with a taper of 2. The taper of each of the two portions is not critical, however, since the mandrel can be positioned during the casting to the precise position required to satisfy the requirements of the tube being cast.

The casting mold assembly includes the graphite liner 22 which is shrunk-fit within a copper sleeve 70. These parts are then placed within the cooling jacket 50 and firmly seated and sealed against water leaks by an asbestos gasket 71 clamped by a retainer ring 72 and bolts 73. The assembly of the cooling jacket 50 and mold (22 and 70) is then press-fitted into socket 74 of guide bearing 20. Water under pressure is fed through a flexible hose (not shown) to inlet nipple 76 of cooling jacket 50, rising vertically along path 77 and entering a narrow annular gap 78 between the copper-graphite die assembly and an inter-battle 79. The water then flows downward at high velocity through the annular space '80 within baflie 79 and impinges on the surface of exiting cast tube 54 at 81. The water is prevented from splashing outwardly by a retaining skirt 82.

Referring now to FIG. 3, there is shown a preferred mandrel of my invention having details for cooling the mandrel. The modified mandrel 23a is inserted in a graphite crucible 12 into which a graphite guide bearing 20 has been press fitted, as described above. The graphite mandrel 23a extends downward through the guide bearing 20 into the casting mold 22, restricting the molten metal to the annular space therebetween to form a cast tube. The mandrel 23a is suspended from a hoist 26 and aligned as described above with respect to the alignment of mandrel 23 shown in FIG. 1a.

-A pin connects mandrel 23a to post 31 shown in FIG. 3. Pin 85 is connected to post 31 by a bolt (not shown) and to mandrel 23a by four radially positioned cap screws 86. Mandrel 23a is internally bored, as at 87, substantially completely through to its tapered end as shown at 37a and is counterbored to a larger internal diameter at the heavy upper section in the portion 88. Axial passages for the water cooling system are provided within the bored cavity of the connecting pin '85 and the cavity of mandrel 23a. This water cooling system comprises a cooling manifold 89 connected to the connecting pin 85 by a transverse bolt 90. Water or other suitable coolant is fed into the cooling manifold 89 through an input pipe 91 and flows downward through a central tube 92, discharging along path 93 into the cooling chamber 94. The coolant is forced back up the mandrel through the annular space within an outer tube 95 surrounding the input tube 92. The returning coolant enters the cooling manifold 89 and is discharged through the exit tube 96. Above the cooling chamber 94, the ouier tube 95 is surrounded by a heat insulating sheath 97 to prevent heat loss in these areas where cooling is not desired.

In operation, the continuous casting of rod (i.e., a solid casting without a hollow center) is iniiially efiected with the lower end of mandrel 23a raised into the upper part of guide bearing by hoist 26. The mandrel 23a is their lowered into the mold to establish tube casting. During the casting, the coolant is circulated through the mandrel to increase removal of heat from the metal being cast, particularly in the portion of the mold adjacent the mandrel cooling chamber 94. The rate of coolant circulation is adjusted in accordance with the conditions of casting, including the nature of the metal or alloy being used, the dimensions of the tube being cast and the rate of withdrawal.

Before the start of casting, the graphite crucible 12 is preheated, as by means of the induction heating coil 17, to temperatures in excess of the melting point of the metal or alloy to be cast. A moderate flow of a coolant, such as cold water, is circulated through jacket 50 to prevent overheating of the mold 22. The graphite mandrel 23 or 23a is fully withdrawn from the mold into the guide bearing 20, and the mold, or the mandrel, or both, as desired, may be vibrated. If both are vibrated they may be vibrated synchronously by the same source, as previously stated, or independently, as required. The molten metal or alloy M is then introduced into the crucible 12 from which it flows through the feed ports 21 in guide bearing 20 into mold 22. A starting cup (not shown), which has previously been placed in the mold, receives and holds the liquid metal until it solidifies. The resulting solid rod, serving only as a starting rod, is withdrawn; and the coolant water flow rate is increased suificiently to keep the temperature from rising while the solid rod is continuously cast from the mold. Casting speeds are then increased to the desired steady-state t-ube casting rate by increasing the speed of withdrawal rollers 61.

The mandrel 23 or 23a is then gradually lowered into the mold to form the continuously cast product into a tube. This transition from rod to tube is made with care to prevent a sudden grabbing of the mandrel by the solidifying metal, causing a possible fracture of the cast. This transition may be facilitated by the use of easily detectable load indications on the strain gauges 29 calibrated according to the particular casting desired.

The zone 67 (FIG. 2) Within the mold, where the molten metal becomes solidified for tube casting, is displaced downwardly from the zone where solidification for easting the solid rod occurs. The position of zone 67 is controlled by the positioning of mandrel 23. If the mandrel is inserted (lowered) beyond the proper position, excessive shrinkage pressure is developed on the mandrel causing transverse checks and cracks on the inside surface of the tube. If the mandrel is insufficiently lowered to the proper position, ripples are produced on the inner surface of the tube.

As the tube is first cast, test cuts on the tube surface indicate deviation in concentricity of the mandrel with respect to the mold. These deviations or error in concentricity can be corrected by adjustments, during the casting operation, of the lateral position of mandrel 23 or 23a in the mold 22 as described above. If irregular surface conditions, particularly the internal surfaces, are detected or observed, the mandrel position is easily adjusted in angular relation to the mold to correct the surface irregularities. The feature of the tapered mandrel permits easy control of the wall thickness of the tube during casting by varying the vertical position of the mandrel. The mold or mandrel, or both, are vibrated as stated according to the desired characteristics of tubing and its material.

With the present invention, it is possible to increase substantially the production rate of tubes continuously cast. The provision for raising and lowering the mandrel, in combination with the continuous feed of molten metal from the pool maintained in crucible 12 above the mold, allows for control of the mandrel variables and enables continuous casting of thin-walled tubing of 4-inch 0D. or less. The means for laterally and angularly positioning the mandrel allows for correction during the casting operation of error in eccentricities. The mandrel being provided with a locally cooled portion in the vicinity of the solidifying zone allows for higher casting speed and improved quality of cast tube. In addition, the means for vibrating the mold and the mandrel, individually or jointly, satisfies a wide range of conditions that are desirable in respect to the preferred grain size and surface characteristics of the various metals and alloys that are used.

I claim:

1. A method of continuously casting tubes which comprises the steps of continuously feeding molten metal into one end of a casting zone while cooling said zone and while discharging a solidified metal product in the form of a rod from the opposite end of said zone, gradually inserting a mandrel into said zone while continuing said feeding, cooling and discharging steps, to form an annular space between said ends of the zone and thereby convert the discharging product from a rod to a tube, and adjusting the position of the mandrel in said zone as the tube is discharging therefrom, to control the character of the discharging tube.

2. A method of continuously casting tubes which comprises the steps of maintaining a pool of molten metal, continuously feeding molten metal by gravity flow from the lower portion of said pool directly into the one end of a casting zone while cooling said zone and while discharging from the other end thereof a solidified metal product in the form of a rod, gradually inserting a mandrel into said zone from said one end thereof while continuing said feeding, cooling and discharge steps, to form an annular space between said ends of the zone and thereby convert the discharging product from a rod to a tube, and adjusting the position of the mandrel in said zone as the tube is discharging therefrom, to control the character of the discharging tube.

3. A method according to claim 2, further comprising vibrating said molding zone axially.

4. A method according to claim 2, further comprising vibrating said mandrel axially.

5. A method according to claim 2, further comprising vibrating said mandrel and molding zone jointly in the axial direction.

6. A method according to claim 2, further comprising vibrating said casting zone axially, vibrating said mandrel axially at a frequency and amplitude different from that of the vibration of said molding zone, and restraining said zone and mandrel against lateral displacement during said vibrations thereof.

7. A method according to claim 4, wherein said mandrel is vibrated at ultra-sonic frequency.

8. A method according to claim 2, further comprising 7 a cooling the portion of said mandrel nearest said lower 2,473,221 6/1949 Rossi 164-83 end of the molding zone. 3,228,075 1/ 1966 Lindemann 16483 XR References Cited FOREIGN TE UNITED STATES PATENTS 5 188,803 11/ 1922 Great Bntaln.

Tama J- Primary Examiner.

2,277,375 3/1942 Tama 164281 XR R. S. ANNEAR, Assistant Examiner. 

